Copper nano-particles, method of preparing the same, and method of forming copper coating film using the same

ABSTRACT

The present invention relates to copper nano-particles having controlled particle size, high monodispersity, and oxidation stability, a method of preparing the copper nano-particles, and a method of forming a copper coating film using the copper nano-particles. The present invention provides a method of preparing copper nano-particles, comprising mixing a copper salt solution with a mixture of a reducing agent and a solvent; where the copper salt solution is added to the mixture at a temperature of 300° C. or less so that the copper salt solution can react with the mixture. It is thus possible to obtain copper nano-particles with controlled particle size and monodispersity by inducing uniform nucleation and nucleus growth through control of the reaction rate and/or the amount of copper ions during synthesis of the copper nano-particles. Moreover, the copper nano-particles of the present invention employ capping organic molecules, which provide oxidation stability by forming a protective dispersant shell around the copper nano-particles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Korean Patent Application No. 10-2006-0011805, filed on Feb. 7, 2006, and all the benefits accruing therefrom under 35 U.S.C. §119, and the contents of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to metal nano-particles, and more particularly, to copper nano-particles, a method of preparing the same, and a method of forming a copper coating film using the same.

2. Description of the Related Art

Generally, nano-particles of a material exhibit characteristics that are different from the characteristics of the bulk material. When the particle size is decreased to nanometer (nm) levels, the nanometer-sized particles have physical, chemical, electromagnetic and optical characteristics which are quite different from those of particles of a micrometer level, due to changes in the electronic and crystalline structures of the material itself or due to an increase in surface energy related to increases in the surface areas of the particles. These nano-particles have a wide range of applications, and therefore nano-particles are actively being studied.

Generally, the melting point of nano-particles is reduced as the size of the crystalline nano-particles is decreased. For instance, the melting point of copper (Cu) is 1,083° C.; however, when the diameter of copper particles becomes 50 nm or less, the surface energy of the particles is rapidly increased as the surface area per unit volume of the particles is increased, and the copper particles begin to fuse with one another even below the melting point. In a liquid phase, in contrast to a solid phase with strong bonding, it is possible to reduce the surface energy by minimizing the surface area of the particles through rearrangement of surface atoms, thus reducing the surface atoms with high energy. That is, the surface atoms intend to reach a stable state by means of reduction of the surface area of the particles. Therefore, the nano-particles can be melted even at a temperature lower than the melting point of an ordinary material.

The characteristics of the nano-particles can be very useful in the case of metals. Recently, there has been activity in developing a flexible device for information display that is light and thin and has high impact resistance by using a substrate based on a flexible plastic film rather than on a conventional hard silicon or glass substrate. With such a display, electric and electronic devices can be fabricated on a plastic substrate. This inevitably requires an electrode formed at low temperature so as to secure thermal stability of a plastic substrate with a relatively low melting point. That is, all processes should be performed at a low temperature that is less than the melting point of the plastic substrate. Since the melting point of a metal can be considerably reduced upon formation of a metal material from nano-particles as mentioned above, such a metal can be applied to a low-temperature process.

BRIEF SUMMARY OF THE INVENTION

An exemplary embodiment of the present invention is to provide metal nano-particles that can be applied using a low-temperature process, and a method of preparing the same.

Another exemplary embodiment of the present invention is to provide a method of forming a metal coating film exhibiting high conductivity using the metal nano-particles by means of heat treatment at a low temperature.

To achieve the above embodiments, the present invention provides a method of preparing copper nano-particles, comprising the steps of preparing a copper salt solution, a reducing agent, and a solvent; mixing the reducing agent with the solvent; and adding the copper salt solution to the mixture at a temperature of 300° C. or less so that the copper salt solution can react with the mixture.

The copper salt solution may comprise copper acetate, copper chloride, copper carbonate, copper nitrate, copper sulfate, or a combination comprising at least one of the foregoing copper salts.

The reducing agent may comprise sodium phosphinate monohydrate, sodium azide, hydrazine hydrate, sodium borohydride, lithium aluminum hydride, or a combination comprising at least one of the foregoing reducing agents.

The solvent may comprise ethylene glycol, diethylene glycol (DEG), triethylene glycol, or a combination comprising at least one of the foregoing solvents.

Mixing of the reducing agent with the solvent may further comprise mixing a dispersant with the mixture of copper salt, reducing agent, and solvent. The dispersant may be capping organic molecule (i.e., an organic molecule having polar and a non-polar end groups) and may comprise polyvinylpyrrolidone (PVP), thioglycolic acid, trioctylphosphine, trioctylphosphine oxide, cetyltrimethylammonium bromide, or a combination comprising at least one of the foregoing dispersants.

The addition of the copper salt solution to the mixture may be performed by injecting the copper salt solution into the mixture using a syringe pump.

The present invention provides copper nano-particles having a particle size of 20 to 300 nm and comprising capping organic molecules adsorbed onto surfaces of the copper nano-particles. The capping organic molecules may comprise polyvinylpyrrolidone (PVP), thioglycolic acid, trioctylphosphine, trioctylphosphine oxide, cetyltrimethylammonium bromide, or a combination comprising at least one of the foregoing capping organic molecules. The copper nano-particles may be prepared by a polyol method.

The present invention provides a method of forming a copper coating film, comprising preparing a coating solution in which copper nano-particles prepared by a polyol method are dispersed; coating the coating solution on a substrate; and heat-treating the coating solution coated on the substrate.

The coating solution may further comprise capping organic molecules adsorbed onto surfaces of the copper nano-particles.

The solvent may comprise a primary solvent including ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, hexylene glycol, glycerol, or a combination comprising at least one of the foregoing primary solvents. The solvent may further comprise a cosolvent including ethyl alcohol, methyl alcohol, acetone, isopropanol, toluene, hexane, heptane, methyl ethyl ketone, ethyl lactate, or a combination comprising at least one of the foregoing cosolvents.

The coating of the coating solution may be performed by means of spin coating, dip coating, droplet casting, inkjet printing or screen printing.

The heat treatment may be performed at a temperature of 200 to 350° C., or may be performed in a vacuum atmosphere, a reduction atmosphere using hydrogen gas, or an inert atmosphere.

The substrate may be a flexible substrate. The copper nano-particles may be prepared by adding a copper salt solution to a mixture of a reducing agent and a solvent at a temperature of 300° C. or less so that the copper salt solution can react with the mixture.

Copper nano-particles can comprise the reaction product of a copper salt, a reducing agent, and a dispersant comprising capping organic molecules, in which the copper nano-particles have a particle size of 20 to 300 nm, and the capping organic molecules are adsorbed onto surfaces of the copper nano-particles.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention are further described in the following description of preferred embodiments in conjunction with the accompanying drawings, in which:

FIG. 1 is a flow chart illustrating processes of a method of preparing copper nano-particles;

FIG. 2 is a SEM (Scanning Electron Microscope) micrograph of copper nano-particles according to an example of a first embodiment;

FIG. 3 is a graph showing XRD (X-ray diffraction) analysis results for the copper nano-particles according to an example of the first embodiment;

FIG. 4 is a SEM micrograph of copper nano-particles according to an example of a second embodiment;

FIG. 5 is a graph showing XRD analysis results for the copper nano-particles according to an example of the second embodiment;

FIG. 6 is a flow chart illustrating processes of a method of forming a copper coated film;

FIG. 7 is a graph showing viscosity characteristics of a coating solution according to an example of a third embodiment;

FIG. 8 is a graph showing resistance characteristics of copper coating films formed according to temperature of a heat treatment process according to an example of the third embodiment; and

FIGS. 9A to 9F are SEM micrographs showing surface microstructures of the copper coating films formed using the heat treatment process according to an example of the third embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, copper nano-particles and a method of preparing the same will be described in detail.

It will be understood in the following disclosure of the present invention, that as used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise”, “comprises”, and “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, components, and combination of the foregoing, but do not preclude the presence and/or addition of one or more other features, integers, steps, operations, elements, components, groups, and combination of the foregoing.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be,interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Copper nano-particles are prepared through a polyol method. The polyol method is a method of synthesizing metal particles by performing chemical reduction reaction at high temperature (i.e., less than or equal to 300° C.) using a reductive polyol solvent as a heat medium, and can improve a reaction rate and a reaction yield as well as prepare nano-sized particles by using a small quantity of a reducing agent and controlling the reaction rate.

FIG. 1 is a flow chart illustrating the processes of a method of preparing copper nano-particles.

Referring to FIG. 1, in an embodiment, a method of preparing copper nano-particles is generally as follows. A copper salt solution for providing copper ions, a reducing agent, a solvent, and a dispersant for dispersing nano-particles are prepared (S10). The reducing agent, the solvent and the dispersant are mixed (S20), and then stabilized through mechanical stirring at room temperature and heated to a temperature of 300° C. or less (S30). The copper salt solution is injected into the mixture heated to the temperature of 300° C. or less (S40), and the resulting mixture is maintained for a predetermined period of time to cause them to react with each other. When the reaction is completed, particles are separated, cleaned and dried (S50) to obtain copper nano-particles.

The method of preparing copper nano-particles will be described in greater detail.

First, a copper salt solution, a reducing agent, a solvent and a dispersant are prepared.

The copper salt solution is a solution for supplying copper ions for use in synthesis of copper nano-particles and comprises deionized (DI) water and a copper salt. The copper salt may include copper acetate, copper chloride, copper carbonate, copper nitrate, copper sulfate, or a combination comprising at least one of the foregoing copper salts.

The mole ratio of a copper salt to deionized water (copper salt/deionized water) in the copper salt solution be 1:2 or less to maintain stability of the copper salt solution. If the mole ratio exceeds 1:2 copper salt to deionized water, reaction stability of the copper salt solution can deteriorate by precipitation of the copper salt.

The reducing agent promotes reduction of copper ions, and may include reducing agents such as sodium phosphinate monohydrate, sodium azide, hydrazine hydrate, sodium borohydride, lithium aluminum hydride, and the like, or a combination comprising at least one of the foregoing reducing agents.

The solvent functions to dissolve the copper salt solution and the reducing agent, and also serves as a medium for nucleation and nucleus growth of copper nano-particles through reduction of the copper ions. Furthermore, the solvent functions to improve the dispersibility of the copper nano-particles, and to control particle agglomeration through adsorption of some functional groups onto the surfaces of the copper nano-particles. The solvent may include ethylene glycol, diethylene glycol (DEG), triethylene glycol, or a combination comprising at least one of the foregoing solvents.

The dispersant is an agent for improving dispersibility, and prevents agglomeration between particles and stabilizes dispersion of the particles. Although the solvent can also function to improve dispersibility and to control particle agglomeration by adsorption onto the surfaces of the copper nano-particles through solvent functional groups as described above, there is a limit to the capacity of the solvent to prevent agglomeration of the nano-particles when the size of the nano-particles is decreased. That is, since the solvent is not dispersing enough to obtain sufficient dispersion stability, an additional dispersant is included.

It is desirable to use capping organic molecules as the dispersant that surrounds the metal particles such that the organic molecules adsorb onto the metal particles. The capping organic molecules may include polyvinylpyrrolidone (PVP), thioglycolic acid, trioctylphosphine, trioctylphosphine oxide, cetyltrimethylammonium bromide, and the like, or a combination comprising at least one of the foregoing dispersants.

The capping organic molecules can improve dispersion stability and simultaneously prevent oxidation of metal particles since the capping organic molecules surround the metal particles in a dense shell. Particularly, in case of copper nano-particles, since copper is a metal that has very low resistance to oxidation by an oxidizing atmosphere, and a copper oxide film reduces electrical conductivity to the level of insulation characteristics, the capping organic molecules are used to prevent oxidation of the copper particles.

Generally, structures and chain lengths of the capping organic molecules vary according to the size of nano-particles, and it is desired to use capping organic molecules that contain a structure having a relatively long chain at one end of the capping organic molecule and a structure having many chains at the opposite end of the capping organic molecule, where the number of chains can be increased as the size of nano-particle crystals increases.

The capping organic molecules and the reducing agent are mixed with the solvent such that they are completely dissolved in the solvent, by mechanically stirring the mixture at room temperature. In order to stabilize the resulting solution, the capping organic molecules may be first mixed with and stirred into the solvent, and the reducing agent may be then added to and again stirred into the mixture.

Next, the mixture of the reducing agent, the solvent, and the capping organic molecules is heated to a predetermined temperature to promote reduction reaction of copper ions. The predetermined temperature is preferably from 100 to 300° C. In general, as heating temperature increases, the reduction reaction is promoted. However, since the reduction reaction is not promoted if the heating temperature is too low, promotion improvement for the reduction reaction saturates with the result that reactants can undergo a change in quality if the heating temperature is too high, and therefore maintaining the foregoing temperature range is desirable.

A copper salt solution is combined with the heated mixture. Here, the copper salt solution can be injected (i.e., metered) into the heated mixture using a syringe pump to control the injection rate of the copper salt solution. That is, the injection rate is controlled using the syringe pump, thereby controlling the amount of copper ions present in the heated mixture, and controlling the reaction rate. The injection rate of the copper salt solution has an influence on the rates of nucleation and nucleus growth, thereby providing control of the particle size and the particle dispersity of the nano-particles. Control of the injection rate can provide monodisperse nanoparticles.

The volume ratio of the copper salt solution to the solvent (copper salt solution:solvent) is preferably 1:4 or less. If the volume ratio of copper salt to solvent exceeds 1:4, there is a concern that the size of particles can undesirably increase during preparation of the nano-particles due to an excess of the copper ions.

After the copper salt solution is injected into the mixture as described above, the resulting mixture is maintained for a period of time sufficient to allow the copper salt to react with the mixture, i.e., the copper salt is reduced. The mixture of the reducing agent, the solvent and the capping organic molecules can have a yellow color. Since copper nano-particles are prepared by a reduction reaction after the injection of the copper salt solution, the color of the mixture gradually changes and the resulting material becomes a dark red color when the reaction is complete.

Upon completion of the reaction, the nano-particles so prepared are separated by centrifugation, and the separated nano-particles are cleaned using methanol, and dried in a vacuum oven.

Spherical monodisperse copper nano-particles having an average particle size of several nanometers to several hundreds of nanometers, e.g., 20 to 300 nm, can be obtained using the aforementioned preparation process. It is possible to obtain monodisperse copper nano-particles with a particle size distribution of ±10%. Here, the particle size distribution of ±10% means ±10% with respect to the average particle size (in nm) of the copper nano-particles.

The method of preparing copper nano-particles according to the present invention is not limited to the foregoing, and various modifications and changes can be made thereto. For example, in another embodiment, although it has been described that the reducing agent, the solvent and the dispersant are first mixed, stabilized and heated and the copper salt solution is then injected into the heated mixture, the method of preparing copper nano-particles is not limited thereto. For example, in another embodiment, it is also possible to mix, disperse and stabilize the copper salt solution, the solvent and the dispersant, then add the reducing agent to the stabilized mixture, and heat the resulting mixture. Furthermore, the aforementioned preparation process may be performed by additionally adding various additives according to desired objects. In this way, copper nano-particles are provided which comprise the reaction product of a copper salt, a reducing agent, and a dispersant comprising capping organic molecules, in which the capping organic molecules are adsorbed onto surfaces of the copper nano-particles.

Process parameters affecting the characteristics of the nano-particles, particularly the size of particles and the dispersity, where copper nano-particles are prepared by the aforementioned preparation method, will be described below.

As mentioned above, when the size of a particle of a material is reduced to nanometer level, these nanometer-sized particles have characteristics that are inherently different from the bulk material, or from larger particles of the material (i.e., a particle size greater than about 1 micrometer) due to changes in the electronic structure of the nano-sized material and the crystalline structure of the material itself, or due to an increase in surface energy correlating with the increase in the surface area of the particles. Specifically, since the melting point of the copper nano-particles decreases as the size of the copper nano-particle crystals also decreases, the copper nano-particles may be used to form a continuous, electrically conducting film by a method using heat treatment at low temperature.

A solution containing the copper nano-particles may be coated onto a suitable substrate and then subjected to heat treatment at low temperature to form a coating film with superior electrical conductivity, wherein the highly monodisperse nano-particles form a conduction path. When the monodispersity of the nano-particles is high, a closed packing structure is constructed by self-alignment of the particles. This is advantageous to form a conduction path during heat treatment. On the other hand, if the monodispersity is low, the particles can have a low packing density in which the voids between the particles are increased in volume. This can cause the conduction path to be easily broken, resulting in low conductivity for the coating.

Therefore, desirable characteristics for copper nano-particles used to form a copper coating film by heat treatment at low temperature are that the size of the copper nano-particles is on the order of nanometers, and that the copper nanoparticles have high monodispersity.

The size of copper nano-particles is regulated by controlling the amount of nuclei formed during nucleation. For a given concentration of copper ions, when the number of nuclei formed during nucleation is small, the amount of copper ions available for the growth of one nucleus is relatively large; and conversely, for the same concentration of copper ions, when the number of nuclei formed in the nucleation step is relatively large, the amount of copper ions available for the growth of one nucleus is relatively small. In other words, as the amount of copper ions for use in growth of one nucleus is increased, the size of the resulting nucleating particles also increases. Therefore, where the same amount of copper ion is used, the size of particles will increase when the number of nuclei formed in the nucleation step is small, whereas the size of the particles decreases when the amount of nuclei formed in the nucleation step is large.

Control over the amount of nuclei formed in the nucleation step as described above can be exercised by controlling the reaction rate, i.e., the rate for the reduction of the copper salt to metallic copper. In general, the reduction rate increases as the reaction temperature is increased, and as the amount (i.e., the concentration) of the reducing agent is increased.

Furthermore, to control the monodispersity of copper nano-particles, it is desirable that nucleation, and the nucleus growth (i.e., growth of the nanoparticles from the nuclei), are clearly divided. To this end, the amount of nuclei formed in the nucleation step and the amount of copper ions to be supplied are both controlled.

If the amount of copper ions consumed by growth of the nuclei formed during nucleation is greater than the amount of copper ions continuously supplied during nucleation, then the concentration of copper ions in the nucleation solution decreases. If the concentration of copper ions is continuously decreased during nucleation to a critical concentration that is less than or equal to the concentration required for nucleation, then nucleation terminates and the nucleus growth begins. Here, if the nucleation step is extended, the final particles grown during nucleation have low monodispersity, since the difference in growth times among generated nuclei increases.

Furthermore, if the amount of copper ions continuously supplied after starting nucleus growth is larger than the amount of copper ions consumed for nucleus growth, then the concentration of copper ions contained in the entire solution increases. If the concentration of copper ions is continuously increased during nucleus growth to a critical concentration that is greater than or equal to the concentration required for nucleation, the process of nucleus growth reverts to nucleation. If nucleation is performed several times in succession, nuclei formed in the respective nucleation steps undergo growth for different periods of time. Thus, the final particles prepared by cycling the nucleation process have low monodispersity.

Thus, the monodispersity of the particles is determined by the reaction rates of nucleation and nucleus growth, and by the amount (i.e., concentration) of copper ions supplied. In addition, if the particles agglomerate with one another, large particles form and the monodispersity of the particles is reduced. The amount (i.e., concentration) of capping organic molecules is therefore useful for mitigating or preventing the agglomeration of the nano-particles.

The size and monodispersity of copper nano-particles are desirably controlled by the reaction rates for nucleation and nucleus growth by adjusting the reaction temperature and/or the amount of the reducing agent. Furthermore, it is desirable to control the amount of copper ions supplied by adjusting the concentration, amount, or injection rate of the copper salt solution. Here, the injection rate can be controlled by injecting the copper salt solution using a syringe pump. Additionally, it is desirable to control the amount of the capping organic molecules.

The particle size and monodispersity of the copper nano-particles can be controlled by adjusting the reaction temperature, the amount of the reducing agent, the concentration, amount, or injection rate of the copper salt solution, and the amount of the capping organic molecules so as to induce uniform nucleation and nucleus growth. Moreover, the use of capping organic molecules with the copper nano-particles prevents oxidation of the copper nano-particles, thereby providing superior oxidation stability.

Hereinafter, the present invention will be described in greater detail in connection with preferred embodiments as described in detail by the following examples.

First Embodiment EXAMPLE 1

8 g of polyvinylpyrrolidone (PVP) dispersant was mixed with 220 ml of diethylene glycol as a solvent and completely dissolved therein through mechanical stirring at room temperature. 1.3518 g of sodium phosphinate monohydrate reducing agent was added to the mixed solution and was completely dissolved therein by mechanical stirring at room temperature. The mixed solution was simultaneously stirred and heated to 200° C. 2.5418 g of copper sulfate pentahydrate was mixed with and completely dissolved in 10 g of deionized water by mechanical stirring at room temperature, thereby preparing a copper salt solution. The copper salt solution was injected into the heated mixed solution at an injection rate of 2 ml/min using a syringe pump. The mixture was maintained at temperature for one hour after addition such that the copper salt solution reacted with the heated mixed solution. Upon completion of the reaction, the resulting particles were separated by means of centrifugation. The separated particles were cleaned four times using methanol and then dried in a vacuum oven to provide spherical monodisperse copper nano-particles with an average diameter of 100 nm.

FIG. 2 is a SEM micrograph of the copper nano-particles prepared according to Example 1 of the first embodiment. As shown in FIG. 2, the copper nano-particles according to Example 1 are formed in a spherical shape and have a particle size of about 70 to 130 nm.

FIG. 3 is a graph showing XRD (x-ray diffraction) analysis results for the copper nano-particles prepared according to Example 1. As shown in FIG. 3, it can be seen that the copper nano-particles are copper nano-particles that do not contain observable impurities such as oxides, i.e., are pure non-oxidized copper nano-particles.

Second Embodiment EXAMPLE 2

16 g of polyvinylpyrrolidone (PVP) dispersant was mixed with 220 ml of diethylene glycol as a solvent and completely dissolved therein by mechanical stirring at room temperature. 1.8587 g of sodium phosphinate monohydrate reducing agent was added to the mixed solution and completely dissolved therein by mechanical stirring at room temperature. The mixed solution was simultaneously stirred and heated to 150° C. Additionally, 2.5418 g of copper sulfate pentahydrate was mixed with and completely dissolved in 10 g of deionized water by mechanical stirring at room temperature, to prepare the copper salt solution. The copper salt solution was injected into the heated mixed solution at an injection rate of 8 ml/min using a syringe pump. The mixture was maintained at temperature for one hour after addition such that the copper salt solution reacted with the heated mixed solution. Upon completion of the reaction, particles were separated by means of centrifugation. The separated particles were cleaned four times using methanol and then dried in a vacuum oven to provide spherical monodisperse copper nano-particles with an average diameter of 40 nm.

FIG. 4 is a SEM micrograph of the copper nano-particles prepared according to Example 2 of the second embodiment of the present invention. As shown in FIG. 4, the copper nano-particles according to Example 2 are formed in a spherical shape and have a particle size of about 20 to 60 nm.

FIG. 5 is a graph showing XRD analysis results for the copper nano-particles according to Example 2. As shown in FIG. 5, it can be seen that the copper nano-particles according to Example 2 are copper nano-particles that do not contain visible impurities such as oxides, i.e., are pure non-oxidized copper nano-particles.

It can be seen from the comparison of SEM micrographs between the first and second embodiments (Examples 1 and 2, respectively) that the copper particles prepared in the second embodiment has a decreased particle size and a higher monodispersity. This can be attributed to the increased amounts of reducing agent and dispersant, and the increased injection rate of the copper salt used in Example 2, even though reaction temperature is lower than that in Example 1.

Thus, particle size and monodispersity can be controlled by inducing uniform nucleation and nucleus growth by controlling reaction temperature; the amount of the reducing agent; the concentration, amount, or injection rate of the copper salt solution; and the amount of the capping organic molecules. Moreover, copper nano-particles can have superior oxidation stability by using the capping organic molecules to prevent oxidation of the copper nano-particles.

In another embodiment, a method of forming a copper coating film according to the present invention is described.

The method of forming a copper coating film is characterized by the use of the copper nano-particles with superior monodispersity and oxidation stability prepared through the aforementioned methods. Since a solution containing the copper nano-particles can be used in a liquid phase material by dispersing them in a specific solvent, it is possible to use liquid phase coating processes to coat the substrate, including spin coating, dip coating, droplet casting, inkjet printing and screen printing, which can be readily performed and in which a pattern can be readily formed. Furthermore, since the copper nano-particles have a low melting point, they are fused at a temperature of 300° C. or less to form a coating film for an electrode by heat treatment at low temperature.

FIG. 6 is a flow chart illustrating an exemplary process of the method of forming a copper coating film according to the present invention.

Referring to FIG. 6, the method of forming a copper coating film generally can be as follows. Copper nano-particles are prepared (S100), and the copper nano-particles are mixed with a solvent and a dispersant (S200). A milling process is performed on the mixture to ensure uniform mixture and dispersion stability of the copper nano-particles, the solvent and the dispersant (S300). The mixture of the copper nano-particles, the solvent and the dispersant is coated on a substrate (S400), and the mixture coated on the substrate is heat-treated at low temperature to form a copper coating film. A copper coating film so prepared has superior conductivity when compared to a copper coating film prepared without the use of a process involving low temperature thermal treatment of monodisperse copper nano-particles.

The method of forming a copper coating film will be described in greater detail.

Copper nano-particles with high monodispersity and oxidative stability are prepared according to the aforementioned preparation process. In this way, the copper nano-particles are synthesized using a polyol process, and the particle size and monodispersity of the copper nano-particles are controlled by inducing uniform nucleation and nucleus growth through control of the reaction rate or the amount of copper ions. Capping organic molecules are used as the dispersant for the copper nano-particles, and prevent oxidation of the copper nano-particles by coordinating to and densely surrounding them to form a protective sphere.

Next, the copper nano-particles, the dispersant and the solvent are mixed to form a coating solution. The amount of dispersant used relative to the weight of copper nano-particles is about 5 wt %. In an embodiment, the coating solution comprises 20 to 50 wt % of solid components including the copper nano-particles and the capping organic molecules, based on the total weight of the coating solution.

A polymer dispersant generally used for increasing dispersion stability can also be used as the dispersant for the copper nano-particles. Since the copper nano-particles of the present invention comprise capping organic molecules adsorbed onto the surfaces of the copper nano-particles so that the capping organic molecules can be used as a dispersant, an additional dispersant may not be used.

The solvent includes a primary solvent and a cosolvent. The primary solvent has relatively high viscosity and a high boiling point, while the cosolvent has relatively low viscosity and a low boiling point. If the solvent has too low viscosity, the solvent will not provide a suitable coating. If the solvent has too high a high boiling point, there is a concern that the solvent will be difficult to volatilize, and will therefore remain after coating and drying of the coating composition. Therefore, a mixture of primary solvent and cosolvent is used to obtain the desired viscosity characteristics for forming a coating film and appropriate drying and volatilization properties.

The primary solvent can comprise ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, hexylene glycol, glycerol, or a combination comprising at least one of the foregoing primary solvents.

The cosolvent may comprise ethyl alcohol, methyl alcohol, acetone, isopropanol, toluene, hexane, heptane, methyl ethyl ketone, ethyl lactate, or a combination comprising at least one of the foregoing cosolvents.

Preferably, the solvent comprises 40 to 80 wt % of primary solvent and the cosolvent as the balance, based on the total weight of the solvent.

Thus also, in a specific embodiment, the coating composition consists essentially of the copper nano-particles, dispersant, primary solvent, and cosolvent as described hereinabove.

Subsequently, stirring is performed to ensure uniform mixture and dispersion stability of the copper nano-particles, the dispersant and the solvent. The stirring process can uniformly mix the coating solution by ball-milling or planetary-milling. Alternatively, the coating solution may be uniformly mixed using sonication.

The uniformly mixed coating solution is coated onto a substrate. The present invention can employ a liquid phase process such as spin coating, dip coating, droplet casting, inkjet printing or screen printing using the coating solution with copper nano-particle dispersed therein. The liquid phase process has an advantage in that expensive equipment is not required and the process is easily performed when compared with a conventional vacuum evaporation process such as sputtering, evaporation or chemical vapor deposition that is performed in a high vacuum atmosphere to form a metal coating film.

After coating of the coating composition, a heat treatment process is executed at low temperature to form a copper coating film. The copper coating film is formed by heat treatment at a temperature selected such that the solvent of the coating solution can be volatilized and the copper nano-particles can be fused. To promote fusion of the copper nano-particles, capping organic molecules adsorbed onto the surfaces of the copper nano-particles should be partially removed. Since the capping organic molecules are thermally decomposed during the heat treatment process, the fusion of the copper nano-particles can be performed without an additional thermal or other treatment process designed to remove the capping organic molecules.

The low-temperature heat treatment process is preferably performed in a non-oxidizing atmosphere to prevent formation of an oxide film on the copper nano-particles. That is, it is desirable to perform the low-temperature heat treatment process under vacuum, in a reducing atmosphere using hydrogen gas, or in an inert atmosphere using nitrogen, argon or helium gas.

Since the copper nano-particles can be fused even at a low temperature of 300° C. or less, the copper nano-particles can be used in the formation of an electrode on a plastic substrate, which essentially requires a low-temperature heat treatment process, i.e., a heat treatment process carried out on a plastic substrate at a temperature that is sufficiently low such that the plastic used is not decomposed, deformed, or distorted upon heat treatment. The copper nano-particles can, in this way, be applied to a flexible device that is light and thin and has high impact resistance, and is prepared from a flexible plastic film. Moreover, since the copper nano-particles have superior electrical conductivity at a very inexpensive unit cost, a high manufacturing process efficiency can be realized.

A copper coating film with superior electrical conductivity can be formed using the aforementioned preparation process.

The method of forming a copper coating film according to the present invention is not limited to the aforementioned preparation process but can be variously modified or changed according to the object and convenience of a process.

Hereinafter, the method of forming a copper coating film according to the present invention will be described in connection with another embodiment.

Third Embodiment EXAMPLE 3

To prepare a coating solution containing copper nano-particles having a solids content of 30 wt %, copper nano-particles with high monodispersity, as prepared according to Example 2, above, were mixed with a solvent containing ethylene glycol as a primary solvent and methyl alcohol as a cosolvent, in a weight ratio of 5:5 of ethylene glycol and methyl alcohol. Capping organic molecules e.g., polyvinylpyrrolidone, which were pre-adsorbed onto the copper nano-particles during preparation of the copper nano-particles according to Example 2, provided the dispersant. At this time, the amount of dispersant used relative to the weight of copper nano-particles is about 5 wt %. The coating solution was uniformly mixed by sonication, and viscosity characteristics of the mixture according to a shear rate were measured to check the dispersity of the mixture. The coating solution was coated on a glass substrate by droplet casting (a liquid phase process), and then the coated glass substrate was subjected to a low-temperature heat treatment process. At this time, in order to form a conductive copper coating film, the heat treatment process was performed under vacuum, at a pressure of 10⁻³ torr in order to prevent formation of an oxide film on the surfaces of the copper nano-particles.

FIG. 7 is a graph showing viscosity characteristics of the coating solution according to Example 3. Referring to FIG. 7, viscosity characteristics of the coating solution are almost constant with an increase in a shear rate. This means that the coating solution has superior dispersion stability.

FIG. 8 is a graph showing resistance characteristics of copper coating films formed according to temperature of a heat treatment process according to Example 3, and FIGS. 9A to 9F are photographs showing surface microstructures of the copper coating films formed according to the temperature of the heat treatment process according to Example 3.

Referring to FIG. 8, it can be seen that the resistance of the copper coating film is decreased according to increase in temperature of the heat treatment process, and the copper coating film exhibits good electrical conductivity even when the copper coating film is formed at a low temperature of 300° C. or less.

FIGS. 9A to 9F show comparison of copper coating films heat-treated at temperatures of 200° C., 225° C., 250° C., 275° C., 300° C. and 325° C. It can be seen from these figures that the particles are closely packed by self-alignment of the particles according to increase in temperature of the heat treatment process. Accordingly, a conduction path is formed and the resistance of the copper coating film is decreased as described above.

Therefore, the present invention can form a copper coating film with superior conductivity by performing a heat treatment process at a temperature of 200 to 350° C. such that copper nano-particles are fused onto a substrate.

The advantages of the copper nano-particles disclosed herein, and the methods for preparing, coating, and heating treating in that expensive equipment is not required and the coating processes to form the copper coating film is readily performed using a liquid phase coating process using a coating solution with the copper nano-particles dispersed therein. Furthermore, since the copper nano-particles can be fused even at a low temperature of 300° C. or less, the copper nano-particles can be applied to formation of an electrode on a plastic substrate, using a non-damaging low-temperature heat treatment process. Thus, the present invention can be applied to a flexible device that is light and thin and has high impact resistance incorporating a flexible plastic film. Moreover, since the copper nano-particles have superior electrical conductivity and are prepared at low costs, increased manufacturing productivity may be realized.

According to the present invention, it is possible to obtain copper nano-particles with controlled particle size and high monodispersity. Moreover, since the copper nano-particles of the present invention employ capping organic molecules, oxidation of the copper nano-particles can be mitigated or prevented. Additionally, a copper coating film can be formed using the copper nano-particles.

Although the present invention has been described in detail in connection with the preferred embodiments, it should not be considered as limited thereto but should be construed based on the appended claims. Further, it will be readily understood by those skilled in the art that various modifications and changes can be made thereto without departing from the scope of the present invention. 

1. A method of preparing copper nano-particles, comprising: mixing a reducing agent with a solvent to provide a mixture; and adding a copper salt solution comprising a copper salt and a solvent to the mixture at a temperature of 300° C. or less such that the copper salt is reduced.
 2. The method as claimed in claim 1, wherein the copper salt comprises copper acetate, copper chloride, copper carbonate, copper nitrate, copper sulfate, or a combination comprising at least one of the foregoing copper salts.
 3. The method as claimed in claim 1, wherein the reducing agent comprises sodium phosphinate monohydrate, sodium azide, hydrazine hydrate, sodium borohydride, lithium aluminum hydride, or a combination comprising at least one of the foregoing reducing agents.
 4. The method as claimed in claim 1, wherein the solvent comprises ethylene glycol, diethylene glycol, triethylene glycol, or a combination comprising at least one of the foregoing solvents.
 5. The method as claimed in claim 1, wherein mixing the reducing agent with the solvent further comprises mixing a dispersant with the mixture.
 6. The method as claimed in claim 5, wherein the dispersant is a capping organic molecule.
 7. The method as claimed in claim 6, wherein the dispersant comprises polyvinylpyrrolidone, thioglycolic acid, trioctylphosphine, trioctylphosphine oxide, cetyltrimethylammonium bromide, or a combination comprising at least one of the foregoing dispersants.
 8. The method as claimed in claim 1, wherein adding the copper salt solution to the mixture is performed by injecting the copper salt solution into the mixture using a syringe pump.
 9. Copper nano-particles having a particle size of 20 to 300 nm and comprising capping organic molecules adsorbed onto surfaces of the copper nano-particles.
 10. The copper nano-particles as claimed in claim 9, wherein the capping organic molecules comprise polyvinylpyrrolidone (PVP), thioglycolic acid, trioctylphosphine, trioctylphosphine oxide, or cetyltrimethylammonium bromide.
 11. The copper nano-particles as claimed in claim 9, wherein the copper nano-particles are prepared using a polyol method.
 12. A method of forming a copper coating film, comprising: preparing a coating solution by dispersing copper nano-particles prepared by a polyol method into a solvent; coating the coating solution on a substrate; and heat-treating the coating solution coated on the substrate.
 13. The method as claimed in claim 12, wherein the coating solution further comprises capping organic molecules adsorbed onto the copper nano-particles.
 14. The method as claimed in claim 12, wherein the solvent comprises a primary solvent comprising ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, hexylene glycol, glycerol, or a combination comprising at least one of the foregoing primary solvents.
 15. The method as claimed in claim 14, wherein the solvent further comprises a cosolvent comprising ethyl alcohol, methyl alcohol, acetone, isopropanol, toluene, hexane, heptane, methyl ethyl ketone, ethyl lactate, or a combination comprising at least one of the foregoing cosolvents.
 16. The method as claimed in claim 12, wherein the coating is performed by spin coating, dip coating, droplet casting, inkjet printing, or screen printing.
 17. The method as claimed in claim 12, wherein the heat treatment step is performed at a temperature of 200 to 350° C.
 18. The method as claimed in claim 12, wherein the heat treatment step is performed under vacuum, in a reducing atmosphere using hydrogen gas, or in an inert atmosphere.
 19. The method as claimed in claim 12, wherein the substrate is a flexible substrate.
 20. The method as claimed in claim 12, wherein the copper nano-particles are prepared by adding a copper salt solution to a mixture of a reducing agent and a solvent at a temperature of 300° C. or less so that the copper salt solution reacts with the reducing agent in the mixture.
 21. Copper nano-particles comprising the reaction product of: a copper salt, a reducing agent, and a dispersant comprising capping organic molecules, wherein the copper nano-particles have a particle size of 20 to 300 nm, and the capping organic molecules are adsorbed onto surfaces of the copper nano-particles. 